Clear to the person skilled in the art, a complete circuit is comprised of multiple separate devices connected to each other through specific electrical pathways. Therefore, every device is manufactured to be isolated but interconnetable during the integrated circuit manufacturing, such that can form a desired circuit structure. With the shrinkage of the dimensions of the semiconductor devices, effects of the STI critical dimensions (CDs) to the electrical properties and yield rate of the devices are becoming ever more critical. Improper isolation may lead to problems such as current leakage, low breakdown voltage, and latch-up effect. Therefore, isolation technology is an important technology in the integrated circuit manufacturing.
Under the sub-65 nm technology node, STI technique is developed and applied in order to improve the circuit performance and device density. As for the STI technique, the CD of the trenches has a profound impact on the electrical properties and yield rate of the devices:
Firstly, as the CDs of the semiconductor devices decrease, the dimension precision of the STI has a more critical effect on the electrical properties of the device. In some areas, a small change in dimension may cause a dramatic change in electrical properties, as shown in FIG. 1.
Secondly, the dimensions of the trenches have a profound impact on the yield rate and ultimate stability of the product, for example, when the dimension of a shallow trench is varied near a limited dimension, the device qualification may sharply decline, even to zero if the dimension exceeds the limit dimension, which leads to scrap of the product, as shown in FIG. 2.
Even though the STI technique is becoming more mature, there still exists several problems in the control of the STI CDs:
{circle around (1)} Generally, the CD of the top of the STI is determined by the linewidth of an active area between two adjacent STI trenches, while the linewidth of the active area is related to the thickness of a bottom anti-reflective coating (BARC) layer formed thereon and the linewidth of a photoresist pattern on the BARC layer. However, under different spin coating speeds, conventional measurement methods can only measure linewidth of the photoresist, which cannot measure thickness of the BARC layer, thus the CD of the shallow trench cannot meet the expected results.
{circle around (2)} During the exposure process, due to the offset of the exposure apparatus itself and other reasons, the linewidth of the photoresist and the coating thickness of the BARC layer may be shifted.
{circle around (3)} During the etching process for forming the STI trench, due to uncertain factors such as variations of atmosphere in the etch chamber or changes in process parameters, the CD of the top of the trenches may deviate from the set value;
{circle around (4)} Especially, when the above mentioned cases both occurred during the lithography and etching process, the root cause of the dimension shifts of the shallow trenches cannot be determined, thus the in-line monitoring and dispatching for the products cannot be achieved, which undoubtedly will produce a huge loss.
In order to solve the above mentioned problems, the industry commonly uses a CD scanning electron microscope (CDSEM) to measure the linewidth of the photoresist pattern. The CDSEM is an electron microscope for forming a scanned image of a sample by scanning an electron beam over the sample surface, detecting secondary electrons generated from the sample, and processing a signal resulting from the detection in synchronism with the scanning of the electron beam.
However, during the practical application, the above monitoring and measuring technique has the following drawbacks:
{circle around (1)} Only one linewidth is measured at a time, which is low efficiency;
{circle around (2)} Since the BARC thickness is not measured, accurate feedback of the actual BARC layer profile cannot be obtained, accordingly, it is incapable to adjust the process conditions based on the feedback information to precisely control the linewidth dimensions of the STI.